LowerTypeTests: Implement exporting of type identifiers.

Type identifiers are exported by:
- Adding coarse-grained information about how to test the type
  identifier to the summary.
- Creating symbols in the object file (aliases and absolute symbols)
  containing fine-grained information about the type identifier.

Differential Revision: https://reviews.llvm.org/D28424

llvm-svn: 292462